1. Field of the Invention
The present invention relates to a process for effecting ultra-deep hydrodesulfurization (HDS) of hydrocarbon feedstocks.
2. Prior Art
In an effort to regulate SO2 emissions from the burning of fuels and to optimise the performance of tail-end catalysts, in particular exhaust treatment catalysts, the regulations as to the sulfur content of fuels, in particular diesel fuels, are becoming more and more strict. In Europe diesel feedstocks will be required as of 2000 to have a sulfur content below 350 ppm, while as of 2005, the sulfur content should be below 50 ppm, with even further decreases not being excluded.
Consequently, there is an increasing need for catalyst systems which can decrease the sulfur content of a hydrocarbon feedstock with a 95% boiling point of 450° C. or less to below 200 ppm, preferably below 100 ppm, even more preferably below 50 ppm, calculated by weight as elemental sulfur on the total liquid product.
In the context of the present specification the term ultra-deep HDS means the reduction of the sulfur content of a hydrocarbon feedstock to a value of less than 200 ppm, preferably less than 100 ppm, and even more preferably to a value of less than 50 ppm, calculated by weight as elemental sulfur on the total liquid product, as determined in accordance with ASTM D4294. The indications Group VIB and Group VIII correspond to the Periodic Table of Elements applied by Chemical Abstract Services (CAS system).
The problem associated with effecting this ultra-deep HDS is that the only sulfur compounds still present in the feed are those which are very difficult to remove. Depending on their source, petroleum fractions can comprise various types of sulfur compounds. In middle distillate fractions, the major sulfur components are benzothiophenes and dibenzothiophenes. In straight-run materials significant quantities of other components are present, such as thiophenes, mercaptanes, sulfides and disulfides. Of these, the sulfides and disulfides are the most reactive, followed by the thiophenes, benzothiophenes, and dibenzothiophenes. Within the group of dibenzothiophenes some components are more reactive than others. In consequence, in conventional HDS, in which the sulfur level is reduced to a value of, say, about 0.3 wt. %, the sulfides and thiophenes are removed. In deep HDS, to a sulfur level of, say, 200-500 ppm, the benzothiophenes are removed. The only compounds remaining then are a limited number of alkylated benzothiophenes, with the alkyldibenzothiophenes which have the alkyl on the 4- or 6-position being particularly difficult to remove.
It has also been found that the reaction mechanisms by which these very refractive sulfur compounds are decomposed is different from that by which the less refractive compounds are decomposed. This is evidenced, e.g., by the fact that the catalysts which are known as particularly suitable for HDS appear to function less well in ultra-deep HDS. For example, conventionally cobalt-molybdenum catalysts are more active in HDS than nickel-molybdenum catalysts. However, for ultra-deep HDS it has been found that nickel-molybdenum catalysts show better results than cobalt-molybdenum catalysts. Reference is made to the paper entitled “Ultra low sulfur diesel: Catalyst and Process options” presented at the 1999 NPRA meeting by T. Tippet, et al.
The consequence of this difference in reaction mechanisms implies that the refiner who is faced with having to produce material with a lower sulfur content cannot just apply his usual hydrodesulfurization catalyst under more stringent conditions. On the contrary, he will have to specifically select the hydrotreating catalyst which is most effective in effecting ultra-deep HDS. This is the more so since the reaction conditions necessary to effect ultra-deep HDS are rather severe in themselves, and the use of a better catalyst makes it possible to select less severe reaction conditions, resulting in energy saving and a longer catalyst lifespan.
We have found that a catalyst which before sulfidation comprises a Group VIB metal component, a Group VIII metal component, and an organic additive is very efficient in reducing the sulfur content of a hydrocarbon feedstock to a value of less than 200 ppm. Additionally, it has appeared that this catalyst makes it possible to effect ultra-deep HDS in combination with improved nitrogen removal, total aromatics removal, and removal of polynuclear aromatics.